1. Field of the Invention
The present invention relates to an ink-jet printer, and especially relates to a star roller used in a paper feed mechanism of the ink-jet printer.
2. Description of the Related Art
In a paper feed mechanism of an ink-jet printer, a roller called a “star roller” is used for conveying a paper sheet in a portion where ink applied on a surface of the paper sheet has not been dried. Since the star roller contacts the surface of the paper sheet, there is a possibility that the ink applied on the surface of the paper sheet is re-transcribed onto a surface of the star roller. Thus, the star roller has a star wheel in which a plurality of contacting portions protrude in radial directions at a predetermined pitch so as to decrease a dimension of each contacting portion. In order to decrease the dimension of each contacting portion of the star wheel to be as small as possible, the thickness of the star wheel in axial direction of rotation of the star roller should be made as thin as possible, and that the top end of each contacting portion should be made as acute as possible.
On the other hand, since the contacting portion of the star wheel is formed at the predetermined pitch, a load of a delivery roller, which is provided for facing the star roller with the paper sheet intervening, is largely varied between a condition that a contacting portion of the star wheel contacts the surface of the paper sheet and another condition that no contacting portion of the star wheel contacts the surface of the paper sheet. Thus, vibrations occur in the paper sheet. When a magnitude of the vibrations becomes larger, conveying speed of the paper sheet facing at an image forming portion facing an ink-jet head also varies, so that quality of the image formed on the paper sheet will be deteriorated. In order to prevent the deterioration of the image quality, it is desirable to make the pitch of the contacting portions of the star wheel as small as possible.
As a manufacturing method of the star roller, a resin molding can be used so that a star wheel and a bearing portion are integrally formed of resin. Alternatively, an outsert molding can be used so that a star wheel made of a metal plate by press working and a bearing portion made of resin are integrally fixed.
Publication Gazette of Japanese Patent Application 10-330005 shows a first conventional star roller in which a star wheel and a bearing portion are integrally formed by resin molding, and the pitch of contacting portions of the star wheel is made smaller. Specifically, a first recess for a first star wheel portion and a half of the bearing portion is formed on, for example, a movable portion of a die, and a second recess for a second star wheel portion and the rest of the bearing portion is formed on a stationary portion of the die. The second recess is formed in a manner so that the pitch of contacting portions of the second star wheel is discrepant by a half pitch with respect to the pitch of contacting portions of the first wheel when the movable portion and the stationary portion of the die are engaged with each other. Melted resin is injected into the recesses of the die, so that the star roller is formed when the resin is solidified. Such a star roller is equivalent to a hypothetical star roller having contacting portions of a star wheel at a half pitch.
Publication Gazette of Japanese Patent Application 2001-80805 shows a second conventional star roller in which two star wheels made of a metal plate by press working are integrally fixed with bearing portion by outsert molding.
In a comparison between the first conventional star roller and the second conventional star roller, the first conventional star roller has an advantage, in that the cost for manufacturing the first conventional star roller is relatively lower than that of the second conventional star roller, since the first conventional star roller is integrally formed by resin molding. The first conventional star roller, however, has as a disadvantage that a thickness of the contacting portion in an axial direction of rotation of the first conventional star roller is thicker than that of the second conventional star roller. Furthermore, a shape of a top end of each contacting portion of the first conventional star roller is affected by flow property of the resin, so that an angle of the top end of the contacting portion in tangential direction of the first conventional star roller becomes a little obtuse than that of the second conventional star roller. Thus, when the first conventional star roller is used in a paper feed mechanism of an ink-jet printer and when ink applied on a surface of a paper sheet has not been dried, the ink could be spread on the surface of the paper sheet due to the contacting portion of the star wheel, or an ink blot could occur due to reattachment of the ink on the contacting portion of the star wheel to another portion on the surface of the paper sheet.
On the other hand, the second conventional star roller has the advantage that the thickness of the contacting portion in the axial direction can be made thinner and the angle of the top end of the contacting portion in tangential direction can be shaped acute than those of the first conventional star roller. The second conventional star roller, however, has a disadvantage that a cost of a die for outsert molding becomes higher, since the die for outsert molding needs a slide core and a mechanism for moving the slide core. Furthermore, the recesses for forming the star rollers must be aligned on only one line in the die, in order to mold a plurality of the star rollers in the same die in one molding operation. Thus, a number of the star rollers molded in one molding operation of the die cannot be increased. As a result, a unit cost of each star roller becomes higher.
Publication Gazette of Japanese Patent Application 2003-145356 shows a conventional method for forming a star wheel by press working. According to the conventional method, a metal plate is preliminary punched out, and contacting portions are shaped using a punch and a die for shear working. Subsequently, the contact portions are made thin and acute by electrolytic polishing. Since the conventional method for forming the star wheel needs many processes, a unit cost of the star wheel becomes higher. Furthermore, a unit const of a hypothetical star roller using the star wheel made of the conventional method in the above-mentioned second conventional star roller becomes much higher, even though the star roller has thin and acute contacting portions.